1. Field of the Invention (Technical Field)
This invention relates to means and methods for determining the porosity and permeability of materials, especially rocks and minerals.
2. Background Art
There are a variety of existing devices for measuring the porosity or permeability of solid substances. Knowledge of a particular material's porosity or permeability may be valuable in a variety of academic and industrial endeavors. A principal field where devices measuring porosity and/or permeability find ready utility is the oil and gas extraction industry. The porosity and permeability of a natural gas or petroleum bearing rock formation have direct and substantial implications for the economical development of the formation. Porosity, which is the fraction of the rock's bulk volume that consists of interconnected porespace, is essential in the evaluation of fluid content of subsurface formations. Information regarding permeability, which is a measure of the rate at which fluid flows through the rock under an applied pressure gradient, is similarly vital for the analysis or design of processes involving subsurface flow.
Consequently, many efforts have been made to provide a device or method for determining these critical material properties of geological formations. Laboratory measurement of these properties usually has been performed by retrieving sample or "cores" from wellbores drilled into the formation. After cleaning, which may involve using hydrocarbon solvents to remove remaining crude oil, the porosity of the dry core is typically determined using a gas expansion technique. Air flow in different directions through the core is commonly measured by enclosing the core in a rubber sleeve equipped with input and output flow ports. Generally, the flow of the test fluid is confined within known boundaries; except for the constant pressure boundaries at which fluid is injected or recovered, the no-flow boundaries are established as such by the elastomeric boot or sleeve pressed against the rock (or occasionally by a plastic coating). From the pressure drop across the core and from flow rate during steady-state flow, the permeability of the sample is then computed by use of equations derived from Darcy's Law.
The non-destructive measurement of permeability on selected small regions of larger samples of rock can be accomplished by use of a flexible-tipped instrument known in the art as the "minipermeameter" or "micropermeameter". The method involved was first described by H. Dykstra and R. L. Parsons under the latter name in "The Prediction of Oil Recovery by Waterflood," Secondary Recovery of Oil in the United States, Chap. 12, pp. 160-174 (American Petroleum Institute, New York, 1950) (presented as Paper #801-24k at the Spring Meeting of the Pacific Coast District, Division of Production, American Petroleum Institute, Biltmore Hotel Los Angeles May 6-7, 1948). Dykstra and Parsons used the perpendicularly-cut end of a soft rubber tube as their probe-tip. These authors investigated the distribution of permeability at the end-face of a core plug, finding significant variability even on this relatively small area.
A somewhat later manifestation of the instrument is described by Morineau, Simandoux and Dupuy in "Etude des heterogeneities de permeabilites dans les milieux poreux," Compte rendu du Ileme Colloque ARTFP, pp. 273-299 (Rueil, May 31-Jun 4, 1965). These investigators showed that there is a significant effect of core heterogeneity on recovery efficiency--this time in solvent displacement experiments.
A major step forward in the rational use of the minipermeameter was taken by David Goggin in his Ph.D. thesis, "Geologically-Sensible Modelling of the Spatial Distribution of Permeability in Eolian Deposits: Page Sandstone (Jurassic), Northern Arizona," Petroleum Engineering Department, University of Texas at Austin, Chapter V, Field Measurement of Permeability, pp. 222-272 (1988), and in an article by Goggin, Thrasher and Lake entitled "A Theoretical and Experimental Analysis of Minipermeameter Response including Gas Slippage and High Velocity Flow Effects," In Situ 12, Nos. 1 & 2, pp 79-116 (1988). In these works, Goggin, et. al. give an analytic treatment of the flow below the probe-tip, complete with numerical simulation to compute the appropriate multiplier by which the permeability can be computed from the flow rate, the pressure in the probe, the dimensions of the probe and the atmospheric pressure and viscosity of air.
Although in principle the minipermeameter is best used on a slabbed surface of infinite extent, the thickness of the slab and the proximity to the probe of other boundaries has obvious effects because the injected air may thus gain alternate pathways to the atmosphere through the extraneous boundaries at the sides or lower side of the slab. Goggin and his collaborators evaluated the influence of both slab thickness and of (cylindrically symmetric) side boundaries, by use of the numerical simulator. As a result of these calculations, it is evident that air flow out of the extraneous boundaries has negligible effect on the measured permeability, so long as these boundaries are further from the probe than about five times the diameter of the tip.
Since Goggin's work, there have been several other versions of the minipermeameter described in the literature. These have included works by Robertson and McPhee in "High Resolution Probe Permeability: An Aid to Reservoir Description," Advances in Core Evaluation Accuracy and Precision in Reserves Estimation, Reviewed Proceedings of the First Society of Core Analysis European Core Analysis Symposium, (London, UK, May 21-23, 1990, Paul F. Worthington (ed.), Gordon and Breach Science Publishers (1990)); and by S. Jones in "The Profile Permeameter: A New, Fast, Accurate Minipermeameter," SPE 24757, pre-print of paper presented at the 67th Annual Technical Conference of the SPE, Washington D.C. (1992). The major motivation for the work cited has been to facilitate the study of local rock heterogeneities. As such, some of the devices reported have been adapted to portable use by geologists for the examination of rock outcrops, and some have been predominately laboratory instruments adapted to make closely-spaced measurements at accurately known positions on the face of slabbed cores or larger rock samples. The general design has also been adapted into commercial instruments.
Supplementing Goggin's numerical work has been that of Zhongming Chen in "Mathematical Basis for Permeability and Porosity Measurements by Minipermeameter," Petroleum Engineering Department, New Mexico Institute of Mining and Technology, Socorro, New Mexico (April, 1992), the teachings of which are incorporated herein by reference, which not only extends the steady-state flow computations, but also computes the transient pressure change inside the rock and at the probe-tip during the brief interval of time after cut-off of the air flow into the entry area defined by the tip. The results of these computations make possible the computation of porosity in a manner described in succeeding paragraphs hereafter.
J. P. Heller has also published a detailed description of a working permeameter which applies many of the principles and features herein disclosed to perform rapid and accurate measurements of samples permeability. J .P. Heller, "The PRRC Automatic Scanning Minipermeameter," PRRC No. 92-20 (June, 1990), available from the New Mexico Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, Socorro, N. Mex., the teachings of which are expressly incorporated herein by reference.
In addition to the academic research reflected in the foregoing references, a number of apparatuses in the field are the subjects of issued patents.
U.S. Pat. No. 4,961,343 entitled, Method for Determining Permeability in Hydrocarbon Wells, to Boone discloses a method of determining the permeability of subsurface geological formations by monitoring the character and content of the drilling mud coming to the surface during oil and gas drilling operations.
U.S. Pat. No. 4,864,845 entitled Electronic Field Permeameter, to Chandler, et al., discloses an electronic field permeameter utilizing mass flow controllers to regulate and measure the flow of gas to the sample and pressure transducers to measure the pressure of the gas applied to the sample. The flow rate and pressure are electronically monitored and processed using microcomputer technology mathematically to calculate permeability. Constant pressure is maintained using a conventional pressure regulator.
U.S. Pat. Nos. 4,649,737 entitled Apparatus and Method for Automatic Testing of Core Samples, and 4,573,342 entitled Apparatus and Method for the Automatic Porosity and Permeability Testing of Multiple Core Samples, to Jones describes a carousel apparatus for testing in seriatim a plurality of core samples. The apparatus is directed primarily toward the automated, mechanized, movement and loading of a large number of samples, and is usable with a variety of instruments for testing permeability or porosity; very few specifics concerning actual permeameter or porometer structure or function are disclosed.
U.S. Pat. No. 4,555,934 entitled Method and Apparatus for Nonsteady State Testing of Permeability to Freeman, et al., teaches an apparatus and method for measuring the permeability of a substance during a non-steady state flow of fluid through a core sample. A change in pressure across the sample is measured over time, and permeability is ascertained as a function of the time-related measure of the pressure change.
U.S. Pat. No. 4,052,885 entitled Portable Device and Method for Determining Permeability Characteristics of Earth Formations, to Shuck discloses an apparatus for determining the direction of maximum permeability in surface rock formations. As apparatus serves primarily to perform comparative qualitative analyses of permeability, the precise means of quantifying permeability is not fully disclosed, but is based on flow discharge measurements.
U.S. Pat. No. 3,839,899 entitled Method and Apparatus for Determining Parameters of Core Samples, to McMillen teaches an apparatus and method for measuring permeability based on changes in pressure across a core sample over measured time.
U.S.S.R. Patent No. 909627 discloses an apparatus for determining the filtration properties of a soil sample based at least in part upon pressure measurements and application of D'Arcy's Equation.
A need remains, however, for an apparatus capable of measuring both the porosity and the permeability of geological core samples, or other materials, that is easy to use, portable, durable, and reliable.